1. Field of the Invention:
The present invention relates to an improvement of a frequency converting circuit employing a phase shift type single sideband signal generation method and is suitable to be formed as an integrated circuit.
2. Description of the Related Art:
It has been well known, as a frequency converting circuit or, in more general definition, a device for obtaining a single sideband signal, to generate a single sideband signal by selectively separating, by filters, single sideband signals of a double sideband signal obtained by suppressing a carrier by means of a multiplier, balanced modulator or a double-balanced mixer or to generate a phase shift type single sideband signal by supplying a pair of a carrier and a signal to a balanced modulator and another pair of a carrier and a signal whose phases are different from those of the first pair by 90 degrees to another balanced modulator and adding an output of one of the balanced modulators to or subtracting the output of the one balanced modulator from an output of the other balanced modulator so that filters are basically eliminated, as disclosed in, for example, B. P. Lathi, "Signals, Systems and Communication", 1965, John Wiley & Sons, B. P. Lathi, "Random Signals and Communication Theory", 1968, John Wiley & Sons and B. P. Lathi, "Communication Systems", 1968, McGraw-Hill.
When a frequency conversion is performed by the frequency converting circuit mentioned above by using a first signal frequency f1 and a second signal frequency f2, a sum f1+f2 or a difference f1-f2 can be obtained easily by a multiplier and a filter if the both frequencies are relatively high and the use of the filter Is allowed. However, when the first frequency is very high and the second frequency is very low, the selective separation by means of the filter is substantially impossible.
The phase shift type single sideband signal generator circuit is used in order to solve the above problem. In this circuit, a filter is basically unnecessary although a simple filter is used practically and, if characteristics and adjustments of the circuit are complete, the desired frequency, f1+f2 or f1-f2, can be selected, so that f1 is suppressed.
However, in a practical circuit, the balanced modulators become easily unbalanced and an error of mixing levels is easily caused in an adder or a subtracter circuit. Therefore, when, for example, f1+f2 is to be selected, it is extremely critical to separate the frequency by a filter, since f1 residing in a proximity of the suppressed carrier and harmonics of the f1-f2 component are undesirably mixed into and are difficult to be removed.
Particularly, when the frequency converting circuit is to be provided as an Integrated circuit, respective balance adjustments must be performed externally of the integrated circuit, for which the number of pins of the integrated circuit must be increased. Thus, the provision of the frequency converting circuit using the phase shift type single sideband signal generating circuit as an integrated circuit is not beneficial.
Now a conventional circuit will be described with reference to FIGS. 11 and 12.
FIG. 11 is a block diagram of a conventional frequency converting circuit and FIG. 12 shows signal waveforms at various points in the frequency converting circuit.
In FIG. 11, it is assumed that an input signal AlcosPt is inputted to an input terminal 101 and an input signal A2cosCt is inputted to an input terminal 105. These input signals are supplied to a .pi./2 phase shifter circuit 103 and a .pi./2 phase shifter circuit 106, respectively. An output of the .pi./2 phase shifter 103 becomes AlsinPt and an output of the .pi./2 phase shifter circuit 106 becomes A2sinCt. In a multiplier circuit (or balanced modulator) 102, the input signal AlcosPt is multiplied with the input signal A2cosCt and, In a multiplier circuit (or balanced modulator) 104, the output signal AlsinPt of the .pi./2 phase shifter circuit 103 is multiplied with the output signal A2sinCt of the .pi./2 phase shifter circuit 106, resulting in the following outputs of the respective multiplier circuits 102 and 104: EQU A1cos Pt.multidot.A 2cos Ct=(A1A2/2){cos (P-C)t+cos (P+C)t}(1) EQU A1sin Pt.multidot.A 2sin Ct=(A1A2/2){cos (P-C)t-cos (P+C)t}(2)
Therefore, an arithmetic operation circuit 107 provides A1A2cos(P-C)t as the output when an addition is performed, or A1A2cos(P+C)t as the output when a subtraction is performed.
Although the signal input to the input terminal 105 is shown as having a rectangular waveform in FIG. 12, the waveform of the input signal is not limited thereto. In a practical circuit, the multiplier circuits 102 and 104 are balanced modulators (double balanced modulators or double balanced mixers) which operate to switch the signal at the input terminal 101 with the signal at the input terminal 105.
In this example, the input signals have rectangular waveforms.
In FIG. 11, the input signal A inputted to the input terminal 101 is supplied to the multiplier 102 and the .pi./2 phase shifter circuit 103 and the input signal C inputted to the input terminal 105 is supplied to the multiplier 102 and the .pi./2 phase shifter circuit 106, as mentioned previously.
In FIG. 12, the multiplier 102 multiplies the input signal A by the input signal C and outputs an output signal E and the multiplier 104 multiplies an output signal B of the .pi./2 phase shifter circuit 103 by an output signal D of the .pi./2 phase shifter 106 and outputs an output signal F.
The arithmetic operation circuit 107 is an adder circuit which adds the output signal E to the output signal F, resulting in a frequency difference as mentioned previously which is outputted as an output signal G. Although the waveform of the output signal G is not exact rectangular, a fundamental wave component signal H is outputted at an output terminal 110 through a low-pass filter (LPF) 109.
The fundamental frequency of the input signal C is 4 times the frequency of the input signal A and the frequency of the output signal H is 3 times the frequency of the input signal A, and this fact confirms a result of calculation of 4 (frequency of input signal C)-1 (frequency of input signal A)=3 (frequency of output signal H).
The waveforms shown in FIG. 12 are ideal. In such circuit construction, when the balance in the balanced modulators 102 and 104 is off even slightly, the waveforms of the output signals E and F become different from those shown in FIG. 12 and the output signal G of the arithmetic operation circuit 107 becomes different from that shown in FIG. 12 when the mixing balance is broken even slightly.
In view of these, it has been desired that a frequency converting circuit which can be used in a wide frequency band from low to high frequencies without requiring a balance adjustment and Is suitable to be constructed as an integrated circuit based on the phase shift type single sideband signal generation method.